Voltage- and calcium-dependent inactivation of calcium channels in Lymnaea neurons

Neurosteroids Alter p-ERK Levels and Tau Distribution, Restraining the Effects of High Extracellular Calcium

Neurosteroids are undeniably regarded as neuroprotective mediators, regulating brain function by rapid non-genomic actions involving interference with microtubules. Conversely, hyperphosphorylated Tau is considered responsible for the onset of a plethora of neurodegenerative diseases, as it dissociates from microtubules, leading to their destabilization, thus impairing synaptic vesicle transport and neurotransmission. Consequently, we aimed to investigate the effects of neurosteroids, specifically allopregnanolone (Allo) and dehydroepiandrosterone (DHEA), on the levels of total and phosphorylated at Serine 404 Tau (p-Tau) in C57BL/6 mice brain slices. In total tissue extracts, we found that neurosteroids elevated both total and p-Tau levels without significantly altering the p-Tau/Tau ratio. In addition, the levels of several enzymes implicated in Tau phosphorylation did not display significant differences between conditions, suggesting that neurosteroids influence Tau distribution rather than its phosphorylation. Hence, we subsequently examined the mitochondria-enriched subcellular fraction where, again, both p-Tau and total Tau levels were increased in the presence of neurosteroids. These effects seem actin-dependent, as disrupting actin polymerization by cytochalasin B preserved Tau levels. Furthermore, co-incubation with high [Ca2+] and neurosteroids mitigated the effects of Ca2+ overload, pointing to cytoskeletal remodeling as a potential mechanism underlying neurosteroid-induced neuroprotection.

Iron overload by transferrin receptor protein 1 regulation plays an important role in palmitate-induced insulin resistance in human skeletal muscle cells

Free fatty acid is considered to be one of the major pathogenic factors of inducing insulin resistance. The association between iron disturbances and insulin resistance has recently begun to receive a lot of attention. Although skeletal muscles are a major tissue for iron utilization and storage, the role of iron in palmitate (PA)-induced insulin resistance is unknown. We investigated the molecular mechanism underlying iron dysregulation in PA-induced insulin resistance. Interestingly, we found that PA simultaneously increased intracellular iron and induced insulin resistance. The iron chelator deferoxamine dramatically inhibited PA-induced insulin resistance, and iron donors impaired insulin sensitivity by activating JNK. PA up-regulated transferrin receptor 1 (tfR1), an iron uptake protein, which was modulated by iron-responsive element-binding proteins 2. Knockdown of tfR1 and iron-responsive element-binding proteins 2 prevented PA-induced iron uptake and insulin resistance. PA also translocated the tfR1 by stimulating calcium influx, but the calcium chelator, BAPTA-AM, dramatically reduced iron overload by inhibiting tfR1 translocation and ultimately increased insulin sensitivity. Iron overload may play a critical role in PA-induced insulin resistance. Blocking iron overload may thus be a useful strategy for preventing insulin resistance and diabetes.-Cui, R., Choi, S.-E., Kim, T. H., Lee, H. J., Lee, S. J., Kang, Y., Jeon, J. Y., Kim, H. J., Lee, K.-W. Iron overload by transferrin receptor protein 1 regulation plays an important role in palmitate-induced insulin resistance in human skeletal muscle cells.

Influence of Ca2+-binding proteins and the cytoskeleton on Ca2+-dependent inactivation of high-voltage activated Ca2+ currents in thalamocortical relay neurons

Ca2+-dependent inactivation (CDI) of high-voltage activated (HVA) Ca2+ channels was investigated in acutely isolated and identified thalamocortical relay neurons of the dorsal lateral geniculate nucleus (dLGN) by combining electrophysiological and immunological techniques. The influence of Ca2+-binding proteins, calmodulin and the cytoskeleton on CDI was monitored using double-pulse protocols (a constant post-pulse applied shortly after the end of conditioning pre-pulses of increasing magnitude). Under control conditions the degree of inactivation (34+/-9%) revealed a U-shaped and a sigmoid dependency of the post-pulse current amplitude on pre-pulse voltage and charge influx, respectively. In contrast to a high concentration (5.5 mM) of EGTA (31+/-3%), a low concentration (3 microM) of parvalbumin (20+/-2%) and calbindin(D28K) (24+/-4%) significantly reduced CDI. Subtype-specific Ca2+ channel blockers indicated that L-type, but not N-type Ca2+ channels are governed by CDI and modulated by Ca2+-binding proteins. These results point to the possibility that activity-dependent changes in the intracellular Ca2+-binding capacity can influence CDI substantially. Furthermore, calmodulin antagonists (phenoxybenzamine, 22+/-2%; calmodulin binding domain, 17+/-1%) and cytoskeleton stabilizers (taxol, 23+/-5%; phalloidin, 15+/-3%) reduced CDI. Taken together, these findings indicate the concurrent occurrence of different CDI mechanisms in a specific neuronal cell type, thereby supporting an integrated model of this feedback mechanism and adding further to the elucidation of the role of HVA Ca2+ channels in thalamic physiology.

Fast Ca(2+)-dependent inactivation of the store-operated Ca2+ current (ISOC) in liver cells: a role for calmodulin

Store-operated Ca2+ channels (SOCs) provide a major pathway for Ca2+ entry in non-excitable cells. SOCs in immortalized liver cells are highly selective for Ca2+ over other cations and are similar to well-studied Ca2+ release activated Ca2+ (CRAC) channels in haematopoietic cell lines. In the present work, employing H4IIE liver cells, we investigated fast inactivation of SOC current (ISOC), which occurs at membrane potentials below -60 mV. This inactivation was significantly reduced when BAPTA, a faster Ca2+ buffer, was used instead of EGTA, and was completely abolished if Na+ was used as a charge carrier in the absence of divalent cations in the external medium. These results suggested that fast inactivation of SOCs in H4IIE cells was Ca2+ dependent and was similar to the fast inactivation of CRAC channels. Experiments showing that the fast inactivation of ISOC was not affected by the disruption of actin by latrunculin B indicate that the cytoskeleton is unlikely to be involved. To elucidate the mechanism of Ca2+ dependence, a possible role of calmodulin (CaM) in SOCs' fast inactivation was investigated. The CaM inhibitors Mas-7 and calmidazolium failed to affect ISOC fast inactivation, whereas over-expression of a CaM inhibitor peptide or a mutant CaM lacking functional EF hands significantly altered the inactivation of ISOC. Out of two exponential components normally required to approximate kinetics of ISOC fast inactivation, the faster component was reduced in amplitude by 30%, compared to the control. The results presented suggest that CaM is responsible for at least part of Ca(2+)-dependent fast inactivation of ISOC in liver cells. It is hypothesized that CaM is tethered to the channel itself and therefore protected from chemical inhibitors.

Effect of hypothalamic proline-rich-polypeptide on voltage-gated Ca2+ currents in retinal ganglion cells

Neurotrophins are molecules that regulate neuronal survival, nervous system plasticity, and many other physiological functions of neuronal and glial cells. Here we studied the physiological action of a novel neurosecretory polypeptide proline-rich polypeptide (PRP), isolated from bovine neurohypophysis neurosecretory granules, on voltage-gated Ca currents and spike firing activity of retinal ganglion cells. PRP reversibly increased high voltage-activated L-type Ca current, but was without effect on low voltage-activated T-type current. PRP also increased the spike after hyperpolarization and reduced the frequency of spike firing, most likely by affecting a Ca-dependent potassium current.

Modulation of Ca2+ currents in rat thalamocortical relay neurons by activity and phosphorylation

Rhythmic low and high frequency activity in thalamocortical networks depend critically on activation of low- and high-voltage-activated (LVA, HVA) Ca2+ currents. In order to test whether Ca2+ currents are modified during repetitive activation, acutely isolated thalamocortical relay neurons of rats, at postnatal days 12 (P12) to P20, were investigated using patch-clamp, Ca2+ imaging and Western blot techniques. High-voltage-activated, but not LVA Ca2+ currents were reduced significantly during 2 Hz stimulation. Ca2+ imaging experiments demonstrated a close correlation between the increase in intracellular Ca2+ levels and the decrease in HVA Ca2+ current amplitudes. Further examination of HVA Ca2+ currents revealed a 'U-shaped' inactivation curve and a time-dependent inactivation process that could be described by a two-exponential function. The 'U-shape' was significantly reduced, current amplitude was increased significantly and time-dependent inactivation revealed a one-exponential decline with Ba2+ as the charge carrier, following activation of the cAMP/PKA pathway, and following application of phosphatase inhibitors (ascomycin, calyculin A). Western blot analysis and the effect of ascomycin indicated an involvement of calcineurin in the inactivation process. Isolation of HVA Ca2+ current components by subtype-specific blockers revealed that changes in time-dependent inactivation, inactivation curve and current amplitude were carried mainly by L-type and N-type Ca2+ currents. Furthermore, Ca2+-dependent inactivation was operative during stimulation protocols mimicking tonic action potential firing. These data indicate a modulation of L- and N-type Ca2+ channels by phosphorylation, resulting jointly in an increased intracellular Ca2+ influx during activity of the ascending brainstem system, the latter occurring during states of wakefulness.